I used G-Strings, a guitar and bass tuner app for Android. It worked pretty well, but I had to hold it down close to the belt. One thing I tried that worked better than expected...if you can hear the pitch but it's too quiet for your tuner app, humm that same pitch into the phone. I know, it sounds dumb, but at those low frequencies the pitch changes audibly as you tighten the belt, so I found it relatively easy to match it, and the tuner app verified this._________________-----------------------------------------
1982 931
1986.5 Silver 928S
2007 Hummer H3 (Daily Driver)

I agree that the 90 degree twist method is pretty vague, unless you've done a whole lot of them and have developed a good feel. The factory workshop manual also cites the 90 degree twist method.

My preference would be to use a tension measurement tool, such as the 9201 tool used for the 924S/944 series cars or something like it. Alas the factory does not provide a spec for the 924 2.0 series cars, at least one I can find. (Just checked the service bulletins, nope.) I wonder whether Continental does.

The harmonic method sounds quite clever! However, the vibration frequency is likely to have nothing to do with tension. Rather, it would normally be a function of the distance between the two pulleys at either end of whatever is vibrating: the belt run. Perhaps I am wrong, and someone on this thread has observed the frequency to change as tension is adjusted. I could be mistaken. Good luck!_________________Kevin
Catellus Engineering, Inc.
http://www.catellusengineering.com/catelluseng@gmail.com

While it is related to length, it is also related to tension. Think about a guitar string. When you tune the pitch up and down, you're changing tension, not length. You only change the length (very slightly) for intonation purposes, so notes up and down the neck are the relative (to open) pitch they should be.

You can audibly hear this with the belt, when you're tightening it. At those relatively low frequencies, there is an easily audible pitch change between relatively small changes in tension.

kevingross wrote:

The harmonic method sounds quite clever! However, the vibration frequency is likely to have nothing to do with tension. Rather, it would normally be a function of the distance between the two pulleys at either end of whatever is vibrating: the belt run. Perhaps I am wrong, and someone on this thread has observed the frequency to change as tension is adjusted. I could be mistaken. Good luck!